A conventional light emitting assembly (referred to as: light engine) includes, three, four or more light emitting components and a control device for operating the light emitting components. A light emitting component can be for example an LED or an OLED. The control device includes a plurality of electronic components. An electronic component may include for example an active and/or a passive component. An active electronic component may include for example a computing, storage and/or regulating unit and/or include a transistor. A passive electronic component may include for example a capacitor, a resistor, a diode or a coil.
A light emitting assembly that is tunable with regard to the color temperature (referred to as: CCT tunable light engine) makes it possible to provide white light having different color temperatures. By way of example, a color temperature of the light which is emitted by a light emitting assembly of this type can be continuously adjusted from warm white to cold white. Depending on the time of day and/or the space in which the light is provided, the desired color temperature can be set by means of the control device.
FIG. 1 shows a chromaticity diagram, e.g. the CIE standard chromaticity diagram. Within the chromaticity diagram, each point is representative of a color of light. A pair including an X-value and a Y-value of the chromaticity diagram corresponds to each point in the chromaticity diagram, wherein the pair can be defined by cX, cY. In the context of this application, one color is defined by exactly one pair of color coordinates. A different pair of color coordinates represents a different color. Consequently, different types of for example white, red, green, and/or blue may exist, each be represented by individual color coordinates and, at least in the context of this application, each be designated as different colors. The color of a radiation emitted by a black body according to Planck corresponds to the temperature of said black body. In the chromaticity diagram, a Planckian locus 20 is defined by the colors which the light of the black body has depending on the temperature thereof. The corresponding temperature is referred to as the color temperature. In the chromaticity diagram, the Planckian locus 20 extends from a warm white color region at the bottom left to a cold white color region at the top right. In the context of this application, white light of different color temperatures is also referred to as light of different colors. In particular, in the context of this application, cold white light has a different color than warm white light. All colors represented by color coordinates which lie on the Planckian locus 20 can be referred to as white, but can be differentiated by their color temperature. Points near the Planckian locus 20 can also correspond to white light, the corresponding light being perceived by a human being generally as white or at least as approximately white, possibly with a hue corresponding to the respective point. By way of example, light of a first color 22 can still be perceived as warm white and light of a second color 24 can still be perceived as cold white. In contrast thereto, light defined by points that are far away from the Planckian locus 20 is no longer perceived as white. By way of example, light of a third color 26 is perceived as green.
If two light sources that emit light of different colors is used for providing light, and the corresponding light is mixed to form a mixed light, then depending on a driving of the corresponding light sources the color of the mixed light can be set within the chromaticity diagram along a straight line connecting the points which correspond to the colors of the light emitted by the individual light sources. By way of example, if one light source provides light of the first color 22 and one light source provides light of the second color 24, the mixed light, depending on the driving of the two light sources, can be set along a first straight line 32 extending from the first color 22 to the second color 24. As an alternative thereto, if one light source provides light of the third color 26 and one light source provides light of the second color 24, the mixed light, depending on the driving of the two light sources, can be set along a second straight line 34. As an alternative thereto, if one light source provides light of the first color 22 and one light source provides light of the third color 26, the mixed light, depending on the driving of the two light sources, can be set along a third straight line 36.
If three light sources that emit light of different colors are used for providing light, and the corresponding light is mixed to form a mixed light, then depending on an individual driving of the corresponding light sources the color of the mixed light can be set within the chromaticity diagram along and within a triangle whose vertices form the three points corresponding to the colors of the light emitted by the individual light sources. By way of example, by means of a light emitting assembly including three light sources that emit light of the first color 22, of the second color 24 and of the third color 26, respectively, depending on the driving of the light sources, mixed light could be generated whose color lies on the triangle or within the triangle whose vertices form the three colors 22, 24, 26.
Nowadays, essentially two approaches are known for realizing light emitting assemblies that are tunable with regard to the color temperature.
In the first approach, three optical channels, that is to say at least three light sources, which emit light of three different colors, are driven independently of one another by means of corresponding three control channels. The light sources are chosen such that the colors of the corresponding light span within the chromaticity diagram a triangle containing part of the Planckian locus 20. An individual driving of the three optical channels by means of the corresponding three control channels then makes it possible to generate light whose color is tunable precisely along the Planckian locus 20. A corresponding driving of the optical channels is complex, however, and a control device comprising suitable three control channels is costly.
In the second approach, only two optical channels, that is to say only two light sources, which emit light of different colors, and only two control channels, for individually driving the corresponding two optical channels, are used for cost reasons. By way of example, a first light source, which emits warm white light, for example light of the first color 22, and a second light source, which emits cold white light, for example light of the second color 24, can be used. By means of driving the two light sources via the corresponding control channels, it is then possible to set the color of the mixed light along the first straight line 32. However, the straight line 32 has only two points of intersection with the Planckian locus 20 and otherwise lies distinctly above or below the Planckian locus 20. Therefore, apart from two exceptions the color of the generated mixed light is distinctly remote from the Planckian locus 20 and the corresponding light is distinctly perceptively not white in wide ranges.